1. Field of the Invention
The present invention is directed to methods for identifying nuclear magnetic spectra from spatially selectable regions of an examination subject which is disposed in a fundamental magnetic field and varying gradient fields as well as being subjected to a sequence of RF pulses for exciting nuclear magnetic resonance in the subject.
2. Description of the Prior Art
Nuclear magnetic resonance devices are known in the art wherein nuclear magnetic resonance signals are obtained from an examination subject which is disposed in a fundamental magnetic field and varying gradient fields, and which has been subjected to a sequence of RF pulses. The resulting nuclear magnetic resonance signals are acquired using a coil disposed in the proximity of the examination region, and spatial selection is achieved by superimposing the transmission/reception characteristic of the coil with a selective slice excitation signal obtained by the combination of a selected gradient field and a frequency-selective RF pulse.
Full topical resolution is generally not employed in nuclear magnetic spectroscopy. The demarcation of individual volume regions of an examination subject, for example an organ, is desired. In a method known from an article by Bottomly et al in the Journal of Magnetic Resonance, Vol. 59, 1984, pages 338-342, the desired volume region is selectively excited for this purpose. The reception as well as the emission of the nuclear magnetic signals occurs by means of a surface coil applied to the examination subject. The reception characteristic and the transmission characteristic of the surface coil are thereby used in combination for the volume selection. When, during the selective excitation, a slice is selected by connecting a magnetic field gradient, a portion of the slice can then be selected on the basis of the characteristics of the surface coil.
This known method, however, presents problems for spectroscopy of compounds having a short relaxation time T.sub.2. This is because the selective excitation necessarily lasts a relatively long time, because an RF pulse having a bandwidth corresponding to the slice thickness must be used. The FID signal, which cannot be interpreted until the end of the excitation, has thus already decayed to a relatively great degree given short T.sub.2 times. This is particularly true for in-vivo phosphorous spectroscopy.
It is known from U.S. Pat. No. 4,021,726 that spatial selection for imaging a selected slice of an examination subject can be achieved by saturation (cancelling) all spins except those in the selected slice. Read-out of the desired information is accomplished using a 90.degree. excitation pulse, which is applied simultaneously with a read-out gradient. This method, however, is not suitable for spectroscopy because the presence of the read-out gradient would destroy the spectral information.